PV Overcurrent Protection

Intermediate

Inside this Article

This array has three source circuits
This array has three source circuits, with an OCPD (aka “series fuse”) for each. Those fuses protect each circuit from potential current from the adjacent strings.

In “Code Corner” in HP159, we discussed Section 690.8 of the 2014 National Electrical Code (NEC)—requirements for circuit sizing and current calculations. The next section—690.9, Overcurrent Protection—was also updated and goes hand-in-hand with 690.8.

Generally, 690.9 requires that conductors used throughout PV systems be protected in accordance with Article 240. Section 690.9 references Article 240 in its entirety, so we need to comply with all rules there—except where Article 690 makes a specific change for PV systems, which occurs twice.

The critical sections on sizing the PV overcurrent protection devices (OCPDs) can be found in Part I, “General Requirements of Article 240.” Section 240.4, “Protection of Conductors,” requires protection of conductors according to their ampacity, as specified in tables in 310.15. However, some modifications as outlined in sections 240.4(A) through (G) may apply to certain PV systems. In 240.4(B), there is an allowance to protect conductors with OCPDs that have amperage ratings greater than the conductor’s ampacity, provided three conditions are met as outlined in 240.4(B)(1)-(3). The first rule in this section does not apply to PV systems, as the rule is directed to conductors supplying power to receptacles.  The second rule stipulates that if the conductor’s ampacity is equal to a standard OCPD’s amperage, it must be matched—you cannot use the next-size OCPD. The final rule is that the OCPD used does not exceed 800 A. To help with this determination, OCPDs are available in “standard sizes” or specific amperage ratings such as 15, 20, and 25 A. The complete list of standard OCPD ratings for both fuses and circuit breakers can be found in 240.6.

To summarize, 240.4(B) allows determining conductor ampacity per 690.8 and protecting conductors with an OCPD that exceeds that ampacity value. For example, after conditions of use are considered, if the ampacity of a 12 AWG THWN-2 conductor equals 17 A, per the NEC, you can protect that conductor with a 20 A OCPD. However, if the calculation results in a conductor ampacity of 15 A, you are required to use a 15 A OCPD, since 15 A is a standard OCPD rating.

Section 240.4(D) covers OCPD requirements for small conductors. See the table for the maximum OCPD size that is allowed for small copper conductors. This is a common Code requirement that applies to all electrical installations, not just to PV. This rule establishes the largest OCPD you can use with these specific conductors, but you should still verify that the conductors used in your PV systems are properly protected per 690.8 and 690.9.

For example, if you apply the required adjustment and correction factors to a 12 AWG conductor and calculate 17 A of ampacity, it is OK to place it on either a 15 A or 20 A OCPD. If that same 12 AWG conductor was corrected down to 14 A, though, you would be limited to using a 15 A OCPD for proper protection. The ampacity tables in 310.15 also reference 240.4(D) for the small conductors to make sure these requirements aren’t overlooked.

Clarifications were added in section 690.9(A) in 2014 that will likely help installers with determining the exact circuit placement for OCPDs. For circuits connected to current-limited devices, PV modules, and the AC output of grid-tied inverters, the OCPD shall be placed at the source of potential overcurrent. The OCPD for source circuits, then, shall be placed at the combiner (i.e. the location where additional current is available from the other series strings). Note: Depending on where the combiner is located, source circuit conductors may run through a building before landing on an OCPD. For a grid-tied inverter output circuit, the OCPD needs to be at the point of utility interconnection—the source of potential overcurrent.

Section 690.9(A) includes an important exception for smaller grid-tied, batteryless PV systems. This is not new to the NEC, but an important section to review. This exception eliminates the need for OCPDs in two specific cases. The first is where there are no external sources connected to the circuit, such as parallel modules or strings, batteries, or backfeed from inverters. In essence, this means you have a single string of modules connected to an inverter with 0 A of backfeed capability. In this scenario, there is no external overcurrent source on the conductors and the inverter cannot convert AC to DC and send it back to the array. Without a source of overcurrent, there is not a need for an OCPD.

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